This invention concerns and relates to improvements in an electrically insulated cable impregnated with insulating oil.
For forming oil-impregnated insulating layers (dielectric insulation) for an oil-immersion type electric cable, electrically insulating paper has heretofore most often been used. Recently, however, polypropylene film in the form of a tape has been used to construct such insulting layers. In addition to far exceeding electrically insulating paper in terms of dielectric breakdown voltage, this film has several advantages such as a low dielectric loss tangent and a dielectric constant approximating the dielectric constant of the insulating oil.
The conventional polypropylene film insulated cable, however, has a disadvantage that swelling with the insulating oil occurs to an exceptionally great extent. Therefore, the use of polypropylene film has entailed various restrictions. For example, when a cable is wrapped in polypropylene film and the resultant polypropylene insulated cable is immersed in insulating oil, the film swells so as to increase the pressure on the core of the cable and consequently deprive the cable of its flexibility and impair the fluidity of the insulating oil between the adjacent insulating film layers. As a measure of avoiding this trouble, the film may be more loosely wound around the conductor. When the film is loosely wound, however, there is a possibility of the film slipping out of place or being wrinkled.
Another disadvantage suffered by the conventional polypropylene film insulated cable is that, where the tapes of the insulating film overlap, the fluidity of the insulating oil between the adjacent films is liable to be low, possibly to the extent of inducing dielectric breakdown because of insufficient roughness of the surface of the polypropylene film.
When polypropylene film is impregnated with any of the alkylbenzene type oils which are preponderantly used as insulating oils in OF cables of the EHV class, the film swells as the temperature of the cable increases so that the film increases in thickness, possibly to the extent of notably increasing the interface pressure between the overlapping plies of film, causing the film to sustain rupture such as due to thermal expansion or contraction of the cable, and moreover forcing upper plies of the film to fall into gaps formed between turns of adjacent lower plies of the film and consequently causing damage. The result is generally degraded electrical characteristics.